IN-9 build 5 + case

Posted on 16 Jan 2012 05:38

ADC input now biased with simple voltage divider and blocking cap. Case doesn't quite close properly, but I have some new, shorter, more closely spec'ed capacitors coming that should fix that. Or, you know, I could put it on a circuit board.


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IN-9 neon bargraph experiment #4: ADC abuse

Posted on 03 Jan 2012 03:32

Add one resistor, one capacitor, one RCA jack, and 200 bytes of firmware to previous blinkenlight demonstration and voilĂ ! The thing I really like about these tubes is that they are so obviously a plasma. You look at them and your mind just registers "electric flame."

Next up: properly connecting the audio input to the ADC

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IN-9 neon bargraph experiment #3: digital output control

Posted on 31 Dec 2011 04:28

My first thought about building a widget around the IN-9 was to use an opamp chip to process an input signal and a timer chip to regulate power. That was what I did for the magic eye VU meter project. Magic eye tubes have an interesting property in that they have a logarithmic response curve, so if you're thinking of making something like a VU meter, that simplifies the requirements for the input circuit. As I looked into such a design for the IN-9, I realized that I not only would I need to add the log scaling (not a big deal), but that I'd also have to add temperature compensation. The expanding requirements got me to wonder if it wouldn't be simpler, and possibly even cheaper to use a small microcontroller instead. Digital logic never needs temperature adjustment.

I pulled out my MSP430 kit and put together a basic circuit, and after a few hours of dicking around with that, I realized that while there are many neat things about the MSP430 family, they are best suited for low-power, low-voltage projects, and are poorly suited to this particular high-voltage project. I managed to kludge something together using voltage regulators and output transistors, but as the circuit got uglier I realized that this was an unsatisfying design direction. So I scrapped it and started over.

My next choice, which turned out to completely exceed my expectations, was to use an attiny85 (I could easily have done this with an attiny25, but the '85 is what I have on hand). Being a committed minimalist, I am delighted by the utter simplicity of this circuit:

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One could think of the modern democratic social contract thusly:

Posted on 13 Dec 2011 20:49

Even those who do not understand prisoner's dilemmas deserve to be protected from them, and everybody benefits when we do.

The reactionary mind rebels against this logic and believes that it will come out ahead in a unconstrained contest. In the overwhelming majority of cases, this belief is mistaken. That is because such a belief is most often based entirely upon emotional intuition and is thus impervious to reason. This is an essential problem of contemporary political economy.

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First working build of magic eye level meter

Posted on 11 Dec 2011 00:13

Now using sooper-kool EM800 tubes instead of frumpy old 6E2 tubes.


Circuit to the left of center is 555-based boost converter. Circuit to the right is LM358-based preamp.

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You must squeeze, Rabban, *squeeeeeze*...

Posted on 07 Dec 2011 00:55

I coaxed another 20% speedup out of the spectrum analyzer for a 3fps gain. The radio module SPI interface is rated for 2MHz, but it turns out you can overclock it a bit, which I achieved by tweaking the OSCCAL register. I see that cringe, but hey, this is very far from a production design! The radio stabilization period is now the unambiguous limiting factor, which means it's unlikely to get any faster :(

Okay, I thought of two more possible ways to speed it up: 1) Switch to a different radio module that might be faster e.g. the TI CC2500. 2) Parallelize the sweep by adding a second radio module. That would complicate the algorithm, but I already know exactly how to do it.

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So many stupid glowy vacuum tubes, so little time

Posted on 02 Dec 2011 10:04


The magic eye project is nearing completion. My little op amp preamp circuit works like a charm for driving the tube from line level. I have a couple of parts on order, but once everything arrives it should all come together very quickly.

So I ordered a batch of IN-9 neon bargraph indicators so I can iterate the design with yet another variety of obsolete communist vacuum tubes.

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The dark season means project time again

Posted on 27 Nov 2011 00:48


Most years I don't like to have too many indoor projects while the weather is good, thought last year I made an exception. In Seattle, prime project season is roughly November-April, and this year I'm kicking it off by finishing some of last year's unfinished (or underdeveloped) projects.

Right now, I'm finishing the magic eye stereo VU meter. Last year I left it just barely complete enough to demonstrate that the tubes work. That meant I got a half-baked power supply working (and then only badly), and I used an external amplifier to drive the audio inputs. This year, I reworked the power supply and I'm adding an internal amplifier so the unit is self-contained and driven by a line level signal. I'm very pleased with how it's turning out, and I think it's going to look stupendously awesome when it's done.

Earlier this month, I reworked the firmware the RF scanner. After living with it for a year or so, the slowness of the refresh rate bothered me and I thought I could do better. This turned out to be an exercise in chasing bottlenecks until I found an external limiting factor. It's not always obvious what the limiting factor will be. In this case it turned out to be close contest between the SPI speed of the radio module interface and the time delay needed for the radio to stabilize before sampling channel strength.

The result is quite satisfying. Refresh rate is now nearly 3x the speed of the old build:

I also plan to finish my IV-4 / IV-17 VFD tube clock, and I have a couple of other project ideas that might be promising. I might even do a straight-up software project for the first time in a while, to celebrate my new Android tablet.

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